In digital communication systems it is common to encode voice data to reduce the bandwidth needed to effectively transmit the voice signal. A great many encoding schemes have been developed and are well known in the art. However, in processing digital telephony signals, a problem has arisen with regard to the detection of ringback signals.
Ringback signals are sent to a calling party so that a tone or tones can be played over the audio output device, typically the earpiece, to indicate to the caller that the called telephone is ringing. To produce a clean ringback signal prior to playing the ringback signal a ringback filter function may be employed. The ringback filter is a simple bandpass filter that passes the ringback tone or tones and rejects the harmonics and sub-harmonics of the ringback signal. To determine when a ringback signal is being received, a detection function may be used. The detection function examines the ringback band. The best prior art ringback detection is performed by using a linear predictive coding (LPC) filter that models the anticipated ringback signal and looks for a match in the received audio signal. If the output of the LPC filter indicates that there is sufficient correspondence to the ringback model in the ringback band, the ringback filter is applied to the signal prior to playing the signal over the audio transducer.
However, in spite of the complexity and sophistication of a LPC filter based detection scheme, false ringback detection still occurs. This happens when received voice data contains discrete frequency components in the ringback band that are similar enough to a ringback signal to fool the LPC filter detection, resulting in the ringback filter being applied to the voice data, causing an undesired distortion of the voice signal. A user may perceive this distortion as an indication of interference, or simply a poor quality product. Therefore there is a need to reduce false ringback detection.